Fig. 3.

Concentration quenching in the NaYF₄:Er@NaYF₄ nanocrystals. a Proposed concentration quenching processes in NaErF₄@NaYF₄ core−shell nanoparticles by 1532 nm excitation. The full, wavy, and colored arrows represent excitation, multiphonon relaxation, and emission processes, respectively. b Emission spectra of NaYF₄:Er (2−100 %)@NaYF₄ nanoparticles in cyclohexane dispersions (0.01 M) as a function of Er³⁺ concentration. All spectra were recorded under excitation of a 1532 nm CW diode laser at a power density of 21 W cm⁻². Inset: luminescence photographs of the corresponding nanoparticle colloids. c The relative oscillator strengths for different excited states as a function of Er³⁺ concentration. Note that the concentrations in the theoretical calculation may not be exactly the same as that used in the experiments but still correctly illustrate the evolution of electronic transition probabilities